1. Field of the Invention
The invention relates to radio-frequency (rf) power generators. More particularly, it relates to rf generators for generating and exciting an inductively coupled plasma (ICP) employed in atomic emission spectrometry.
2. Description of the Prior Art
Inductively coupled plasmas (ICP) have been used to excite samples for analytical emission spectroscopy. The paper "Automatic Multi-Sample Simultaneous Multi-Element Analysis with a M.F. Plasma Torch and Direct Reading Spectrometer" by S. Greenfield, I. L. L. Jones, H. MCD. McGeachin and P. B. Smith, published in Analytica Chimica Acta; 74 (1975); discusses an ICP coupled with a 30-channel direct reading spectrometer with fully automatic sequential sampling, exposure and read-out.
The paper "A Stabilized R.F. Argon-Plasma Torch for Emission Spectroscopy" by P. W. J. M. Bowmans, F. J. deBoer and J. W. Ruiter, published in Philips Technical Review (1973); discusses a rf generator system for producing an inductively coupled argon plasma (ICAP). The system of Boumans, et al, is adapted to provide stabilized power to the ICAP and minimize plasma intensity variations which occur when a sample is introduced into the plasma.
In conventional excitation systems, such as those of Greenfield, et al. and Boumans, et al., a rf generator ordinarily provides power to combined tuning-work coil. This coil operates both as the inductor coil in the output tuning (tank) circuit of the generator and as the plasma producing work coil. The plasma is typically annular in shape, providing a tunnel region into which a sample is introduced for excitation.
Conventional exciter apparatus and systems have been adequate when the plasma is established and operating. However, during the critical periods of startup and plasma ignition, such prior systems typically operate in an unstable region of their operational envelopes. Complex controls have been required to closely regulate the power into the tuning-work coil to ensure reliable plasma ignition. As a result, the prior devices have been expensive, complex and bulky and have ordinarily required complicated three-phase power.
When generating power at radio-frequencies, the radiated power from the generator may be closely regulated and the AC frequency of operation kept within a specific allowed bandwidth to prevent rf interference with other devices, such as nearby communications equipment. Typically, the AC frequency has been substantially fixed by use of a crystal controlled oscillator. Generators with crystal controlled rf oscillators, however, typically require additional amplifier stages to develop the output power needed to produce and sustain an ICP, and often employ special rf transmission cables, rf connectors and associated impedance matching circuitry. In addition, special, complex tuning adjustment circuits have been required to compensate for resonant frequency shifts that occur in the output tuning circuits during periods of plasma ignition and plasma excitation of an analytic sample. During such periods, the rf power output tuning circuit becomes mismatched from the fixed oscillator frequency. This changes the power delivered into the tuning-work coil and causes fluctuations in the plasma intensity. The plasma may even extinguish. To compensate for this problem, complex circuits have been employed to closely regulate power output, voltage phase relations and resonant frequencies of the output tuning circuits to ensure adequate power into the plasma.
Radio-frequency generators which employ a free-running oscillator have generally been preferred because they are simpler and more economical than generators with fixed frequency oscillators. However, ordinary exciter systems using such generators experience very large frequency shifts sweeping over hundreds of kilohertz, particularly during plasma ignition. As a result, conventional generators with free-running oscillators exceed allowable operational bandwidths and have required bulky and costly rf shielding to prevent disruptive rf interference with other equipment.
Thus, these conventional plasma exciter apparatus have remained complex, expensive and bulky and have generally required complicated power supplies. Apparatus in which the generator output frequency is closely controlled have required additional amplifiers, additional transmission components and complicated control circuitry, particularly during plasma ignition, to regulate power into the plasma. Apparatus in which the rf generator employs a free-running oscillator have exhibited excessive frequency shifts and required substantial rf shielding. Because of their complexity, bulk and high cost, these conventional plasma excitation devices have been unsuitable for use in small office-type laboratories.